The present invention relates to a tape recorder using DC erasure system and more particularly to such a tape recorder capable of utilizing an AC erasure unit, if necessary.
There are two types of systems for erasing signals recorded on a magnetic tape. One is a DC erasure system employing the unidirectional residual magnetism of a magnetic substance for applying a saturating DC magnetic field to a magnetic tape. The other is an AC erasure system involving convergence of residual magnetic flux into zero value by applying a saturating AC magnetic field to the magnetic tape. The DC erasure system has the advantage of simple construction and good efficiency, and if a permanent magnet is used as a magnetic head, the power consumption during erasure is zero. Hence, the DC erasure system is usually adopted for a tape recorder in which a battery serves as a power source.
FIG. 1 shows a tape recorder incorporating a DC erasure system. In FIG. 1 a microphone 1 is coupled to a magnetic recording and reproducing head 6 through a microphone amplifier 2, a recording and reproducing change-over switch 31, a recording amplifier 4 and a bias trap 5. The magnetic head 6 is coupled to ground through a change-over or selector switch 32 which is interlocked with the change-over or selector switch 31, and also coupled to ground through a reproducing amplifier 7, the selector switch 31, a volume control 8, a power amplifier 9, an earphone jack 10, a loudspeaker 11 and a selector switch 33 which is also ganged with the selector switch 31. The magnetic head 6 is connected to a DC power source Vcc through a bias oscillator circuit 12, and a change-over or selector switch 34 which is also interlocked with the switch 31. The DC source Vcc is connected to a magnetic erasing head 13 through the switch 34. Tape selector switches 141, 142, and 143 are switched to change contacts in accordance with the kinds of magnetic tape to be used.
In the reproducing or playing mode, the switch 31 connected the reproducing amplifier 7 to the speaker 11 through the volume control 8, the power amplifier 9 and the earphone jack 10. The switch 32 grounds the junction of the magnetic head, the bias trap 5 and the bias oscillator circuit 12. The switch 33 is closed to apply the current signals picked up from the magnetic head 6 to the speaker 11. At the same time, the switch 34 is opened to prevent DC power from being supplied to the bias oscillator circuit 12 and the magnetic erasing head 13. In this reproducing mode output signals reproduced from the magnetic head 6 are amplified by the reproducing amplifier 7 and the power amplifier 9 and supplied to the speaker 11 which generates reproduced sound. The magnitude of the reproduced sound can be adjusted by the volume control 8. If an earphone (not shown) is connected to the earphone jack 10 the sound can be heard through the earphone instead of the speaker 11.
In the recording mode the switch 31 connects the microphone 1 to the recording amplifier 4, the switch 32 connects the junction point the magnetic head 7 to ground, the switch 33 is opened to disconnect the speaker 11 from, and the switch 34 is closed so that the DC source Vcc is connected to the bias oscillator circuit 12 and the magnetic erasing head 13. In this recording mode when sounds are applied to the microphone 1 its output is amplified by the microphone amplifier 2 and the recording amplifier 4 and supplied to the magnetic head 6 as a recording signal current through the bias trap 5. At the same time, the recording head 6 receives bias current from the bias oscillator circuit 12. The output of the microphone 1, therefore, is recorded onto a magnetic tape (not shown) by the head 6. In this case the erasing head 13 receives the DC erasing current from the DC source Vcc so that the signals which have been recorded on the magnetic tape are erased before recording new signals. Such a tape recorder perform temporary recording and reproduction operations.
However, this tape recorder generates a large number erasing noises which arise from the DC erasing system. The uneven magnetic layer of the tape and the uneven contact condition of the magnetic head to the magnetic tape cause uneven residual magnetic flux density. This results in an appearance of noise which severely degrades the quality of the reproduced sound. Moreover, the recorded or reproduced signal contain large distortions as compared with signals processed by an erasing system. Because of these disadvantages, a DC erasure system limit the application of a recorder to a narrow scope.
A portable tape recorder utilizes a battery as a power supply source. However, indoors, where power lines are available, one can use a transducer composed of a transformer and rectifier for converting the commercial supply voltage into a DC voltage as the power source of the tape recorder.